The following is not to be construed as an admission of the common general knowledge.
Synthetic polymeric ultrafiltration and microfiltration membranes are known for a variety of applications including desalination, gas separation, filtration and dialysis. The filtration properties of the membranes depending upon features of membrane morphology such as symmetry, pore shape, pore size and on the chemical nature of the polymeric material used to form the membrane.
Microfiltration and ultrafiltration are pressure driven processes and membrane pore size correlates with the size of the particle or molecule that the membrane is capable of retaining or passing. Microfiltration can remove very fine colloidal particles in the micrometer and submicrometer range, down to 0.05 μm as a general rule, whereas ultrafiltration can retain particles as small as 0.01 μm and smaller.
Membrane gas filtration or gas transfer is also possible, allowing separation of dissolved gas from liquids. The process occurs by gas diffusion and forms the basis of, for example, transmembrane distillation, osmotic distillation, degassing, drying and adsorption. Reverse processes, such as bubbleless membrane aeration, and those processes which occur in membrane contactors, are also possible.
The chemical properties of the membrane are also highly important in the case of gas transfer membranes.
Liquids can be prevented from passing through a porous membrane given the correct combination of    i) a suitably small pore size and    ii) the required combination of hydrophilicity/hydrophobicity of the membrane and the liquid.
Thus, the size of the liquid molecules is not the limiting factor, but rather passage through the membrane is determined by the balance of liquid-liquid and liquid-membrane forces.
Gases, on the other hand, have no such problems passing through a membrane as they have no surface tension, and so will pass through a membrane pore of any size larger than the size of the molecules (which is typically of the order of 0.1 nm), provided the diffusion conditions are appropriate (ie gas will not diffuse into a region of higher pressure) and provided there is no adsorption onto the membrane wall.
Hydrophilic liquids (such as aqueous solutions) will not pass through hydrophobic membranes with a small pore size, and nor will hydrophobic liquids pass through hydrophilic membranes which have a small size. Water for instance has a very small molecular size, but requires high pressure to pass through small pores in hydrophobic membranes due to surface tension. As pore size of hydrophobic membranes decreases, greater pressures are required to force water through the membrane.
Water can be forced through a hydrophobic membrane by use of sufficient pressure, but the pressure needed is very high, 150-300 psi for micro-ultrafiltration membranes. Membranes are likely to be damaged at such pressures and in any case generally do not become wetted evenly. Accordingly, when used in water filtration applications, hydrophobic membranes need to be hydrophilised or “wet out” with agents such as ethanol or glycols to allow water permeation. Gas filtration membranes of course are better if the membranes are not wetted out and the hydrophobic nature of the polymer is retained.
Gas filtration is often required in processes which employs very severe conditions. For example, it is useful in the electronics industry, for the stripping of HF gas or the degassing of caustic solution prior to use, or in the area of chlorine/alkaline electrolysis where a membrane needs to withstand hot concentrated caustic or acid in combination with chlorine. Many conventional materials used in membrane fabrication are unable to withstand such high levels of chemical attack, even if they can be formed in a manner which produces pores of a suitably small size.
Even degassing tap water to remove low concentrations of dissolved chlorine used to kill bacteria can expose membranes to large amounts of chlorine over the working life of a membrane by virtue of the high throughput. Eventually, the membrane can exhibit yellowing or turn brittle, signs of degradation of the membrane.
Currently, poly(tetrafluoroethylene) (PTFE), polyethylene (PE), polypropylene (PP) and poly(vinylidene fluoride) (PVDF) are the most popular and available hydrophobic membrane materials.
PVDF exhibits a number of desirable characteristics for membrane applications, including thermal resistance, reasonable chemical resistance (to a range of corrosive chemicals, including sodium hypochlorite), and weather (UV) resistance.
While PVDF has to date proven to be the most desirable material from a range of materials suitable for microporous membranes, the search continues for membrane materials which will provide better chemical stability and performance while retaining the desired physical properties required to allow the membranes to be formed and worked in an appropriate manner.
The limitation of gas filtration membranes has been their poor stability in very harsh chemical environments and at elevated temperatures. The search continues for membrane materials that will provide better chemical stability and performance while retaining the desired physical properties required to allow the membranes to be formed and worked in an appropriate manner.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to at least provide a commercial alternative.